1. Field of the Invention
The present invention relates to a hollow foam molding having a polypropylene resin foam layer and to a blow molding method for producing such a hollow foam molding.
2. Description of Prior Art
A foam blow molding process has begun to be used in recent year for the production of hollow moldings having a foam layer. Thenceforth, various techniques have been proposed for the foam blow molding process. A typical such technique includes the steps of extruding a foamable melt containing a base resin and a blowing agent through a die to obtain a foam parison, placing the parison in a mold, and blowing a gas into the parison to blow mold the parison (International Publication No. WO99/28111 and U. S. Published Patent Application No. 2004-62885).
The hollow foam moldings obtained by such a process are light weight and excellent in heat insulation property and may be utilized for various applications such as receptacles, ducts, automobile parts and electric appliance parts. Especially, because hollow foam moldings of a polypropylene resin have excellent heat resistance and rigidity, they are expected to be utilized as air conditioner ducts for automobiles.
Conventionally, air conditioner ducts of a non-foam resin produced by blow molding have been used in automobiles. In spite of the fact that only a limited space is available in automobiles for mounting the air conditioner ducts, the automobile ducts are required to have a reduced air flow resistance in order ensure a required level of air flow capacity. Thus, air conditioner ducts must have a thin wall thickness.
No methods are currently known in the art to produce, by blow molding, a light weight air conditioner duct which is made of a polypropylene resin foam and which has a thin wall thickness. It is known to produce a hollow foam molding of a polyolefin resin foam by blow molding using a chemical blowing agent (U.S. Pat. No. 5,714,227). The molding obtained with this method, however, has an apparent density of above 0.7 g/cm3 and is insufficient in lightness in weight, though the wall thickness thereof is thin. When the chemical blowing agent is used in a large amount, a hollow foam molding of a polypropylene resin foam having a good appearance and an apparent density of below 0.7 g/cm3 cannot be obtained. Further, the closed cell content of such a foam molding is disadvantageously reduced.
It is also known to produce a hollow foam molding of a polypropylene resin foam by blow molding using a specific polypropylene resin having a high melt tension and a physical blowing agent such as butane (International Publication No. WO99/28111). The use of the physical blowing agent can produce a light weight hollow molding having an apparent density of 0.3 g/cm3 or less. However, the wall thickness is so large that it is difficult to use the molding as an air conditioner duct for automobiles. In this case, when a die lip having a small clearance is used for producing a parison with a thin wall thickness by extrusion, corrugation unavoidably occurs so that the wall thickness of the hollow molding obtained from such a parison becomes non-uniform. With this method, therefore, it is impossible to obtain a light weight hollow foam molding having a thin wall thickness.
The known hollow foam molding of a polypropylene resin is thus either a thick-walled molding having a small apparent density or a thin-walled molding having a large apparent density. It is impossible with the known technique to produce a hollow foam molding of a polypropylene resin having a thickness of 1.0 to 5.0 mm, an apparent density of 0.35 to 0.65 g/cm3 and a uniform thickness.
In the case of extrusion molding, it is possible to produce, without difficulty, a polypropylene resin foam sheet having a thickness of about 1.0 mm, as disclosed in U.S. Pat. No. 5,180,751 and Japanese Unexamined Patent Publication No. 2004-359917. Namely, such a thin foam sheet may be obtained by drawing a uniform, relatively thick extruded foam sheet while cooling during the hauling thereof rather than by using a die lip having a small clearance. The technique in the above extrusion molding cannot be applied to the blow molding, because a parison cannot be drawn in the extrusion direction beyond a certain level and because the parison must be molded in a succeeding blow molding step while it is in a softened state. Namely, it is impossible to draw a thick-walled parison into a thin-walled one.